The Planting Date Conundrum for Corn

Early planting favors higher yields, but does not guarantee higher yields.

Statewide averages for planting date and yield are not strongly related.

Planting date is but one of many yield influencing factors.

onventional
wisdom says that the prime planting window to maximize corn yields in much
of Indiana opens about April 20 and closes about May 10. This “window” typically
opens about one week later across the northern tier of Indiana counties (later warmup) and about one week earlier across the southern tier of Indiana
counties (earlier warmup).

Very little corn, if any, has been planted in Indiana or elsewhere across the Corn Belt as of 21 April (USDA-NASS, 2013). By itself,
this is not much cause for concern because typically only a relatively small percentage
of acres is ever planted by this date in Indiana. However, the specter of
seriously delayed planting is clearly on the horizon because little other spring fieldwork
has been completed due to the frequent and sometimes excessive rainfall in
recent weeks. For some growers, tillage operations, herbicide applications,
and nitrogen fertilizer applications must be completed first before they can
consider planting their crops.

What are the consequences of a delayed start to planting? How important a
predictor of statewide corn yield is planting date anyway? Does late planting
in and of itself guarantee lower than normal yields? Good questions, but the effect of planting
date on statewide average corn yield is not clearcut.

If one reviews USDA-NASS crop progress reports for the past 20 years (USDA-NASS,
2013), there is NOT a strong relationship between planting date and absolute yield
on a statewide basis for Indiana. Specifically, departures from annual trend yields are
not strongly related to corn planting progress. Figures 1 and 2 illustrate
this relationship for two measures of planting progress; percent acres planted
by April 30 or by May 15. Even though one can statistically define a mathematical relationship between
departure from trend yield and planting progress by April 30 or May 15, the
relationship only accounts for 22 to 24% of the variability in yield trend departures from
year to year. In other words, a number of yield influencing
factors (YIFs) in addition to planting date also affect the ultimate absolute yield
for a given year.

Here's the Conundrum

Why is it that every corn agronomist known to man preaches
about the importance of timely planting and yet the statewide statistical
data suggest that planting date accounts for only 23% of the variability in
statewide yields from year to year? Let's look more closely about this seeming
conundrum.

It is true that corn grain yield potential declines with delayed
planting after about May 1 (Myers & Wiebold, 2013, Nafziger,
2008; Nafziger, 2011). Estimated yield loss per day with delayed planting varies from about 0.3% per day early in May
to about 1% per day by the end of May. Yield potential goes down with delayed
planting because of a number of factors, including a shorter growing season,
greater insect & disease pressure, and higher risk of hot, dry conditions during pollination.

However, the good news is that planting date is only one of many YIFs for
corn. What is important to understand is that yield loss due to delayed planting
is relative to the maximum possible yield in a given year. In other words,
if all the other YIFs work together to determine that the maximum possible
yield this year for the optimum planting date is 220 bu/ac, then the consequence of a 10-day planting delay
beyond April 30 (at 0.3% decrease per day) would be a yield potential of about 213 bu/ac (i.e.,
220 bushel potential minus [10 days x 0.3%] due to delayed planting). However, if all
the other YIFs work together to determine that the maximum possible yield
this year for the optimum planting date is only 150 bu/ac, then the consequence of a 10-day planting delay
beyond May 1 (at 0.3% decrease per day) would be a yield potential of about 146 bu/ac (i.e.,
150 bu/ac potential minus [10 days x 0.3%] due to delayed planting). Make sense?

Consequently, it is possible for early-planted corn in one year to yield
more than, less than, or equal to later-planted corn in another year depending
on the exact combination of YIFs for each year. Figure 3 illustrates this often confusing concept. In that graph, a delayed planting of corn (B) in an otherwise high yielding year may still be higher yielding than a crop planted on the optimum planting date (C) in an otherwise lower yielding year. Farmers know this to be true because some have had June-planted crops in recent years that ultimately yielded better than any crop they have ever had.............. because the remainder of the growing season following the delayed planting was exceptional.

For example, the crop years 2012 and 2009 represent early and late planting
date years in Indiana. About 94% of the state's crop was planted by May
15 in 2012, but only 20% of the crop was planted by May 15 of 2009 (Fig. 2). Yet,
the earlier planted 2012 crop yielded 38.7% BELOW trend yield for that year
and the later planted 2009 crop yielded 9.3% ABOVE trend yield. Why? Important
differences in YIFs between the years other than simply the planting dates.

Bottom Line

Let's not succumb quite yet to fearmongering triggered by the prospects of
a delayed start to corn planting in 2013. “Mudding in” a crop early to avoid
planting late will almost always end up being an unwise decision. While important,
planting date is only one of many yield-influencing factors for corn.

Another reason that it is probably too early to fearmonger about the anticipated
late start to planting is that growers have the machinery capacity to "catch
up" quickly once the weather and soil conditions become favorable for
planting. The 1992 planting season began as one of the slowest but
quickly recovered within two weeks to a respectable pace and finished
the season with the largest POSITIVE departure from trend yield in the past
20 years (Fig. 4). We also know from past years' experiences that, on average, 50%
of the state's corn crop is typically planted over about an 21-day period
(Fig. 5). Furthermore, it is not unheard of for growers to plant 45 to 50%
of the state's crop in a single week given good working conditions (Fig. 6).